Toolchains are a central concept in Buck2, since they are supported in the core rather than just being something defined in the prelude, but they are essentially not documented: it is not listed under Concepts in the sidebar, not part of the Glossary of Terms, and there is little information about what a toolchain really is or does.

Some questions that would be worth answering:

• What does Buck2 call a toolchain and what is it used for?
• What purpose does rule's is_toolchain_rule serve?
• When should one write a rule versus a toolchain?
• How can/should one use an existing toolchain?
• How do toolchains relate to configuration/constraints?

I have a vague notion of "toolchains let you decouple a rule from the actual compilers, runtimes, sysroots, etc. that it might use", but that's not enough for me to write good documentation about them I'm afraid...